1. Field of the Invention
This invention relates generally to coal-fired electricity generating plant design, and more particularly to a transport oxy-combustor. The transport oxy-combustor is used for the combustion of coal with oxygen as oxidant to generate a substantially pure CO2 stream after condensing moisture out of the flue gas.
2. Description of Related Art
Oxy-combustion is part of coal-fired electricity generating plant design that has the potential for significantly reduced CO2 emissions compared to conventional coal power plant designs. In oxy-combustion, coal is combusted in an enriched oxygen environment using substantially pure or substantially pure oxygen diluted with recycled flue gas. From this process, the flue gas is composed primarily of CO2 and H2O, so that a concentrated stream of CO2 is produced by simply condensing the water in the exhaust stream. An advantage of oxy-combustion over air-fired combustion is that it provides a high potential for a major reduction in both CO2 separation and capture costs because virtually all of the exhaust effluents can be captured and sequestered.
U.S. Pat. No. 6,505,567 to Anderson et al. discloses a method of operating an atmospheric circulating fluidized bed that feeds substantially pure oxygen as oxidant to combust the fossil fuel in the combustor. A part of fine solids entrained by flue gas is cooled in an external fluidized bed heat exchanger and recycled to the lower portion of the combustor. The solids cooling process generates a small fraction of steam for power generation. The small portion of entrained, cooled solids recycled can aid in controlling the combustor temperature.
Anderson et al. also discloses recycling sufficiently large amounts of the gaseous combustion product to the combustor to control the combustor's temperature. The method of operating such a circulating fluidized bed combustor (CFB) is essentially the same as a conventional CFB combustor, except replacing air with oxygen as the oxidant. However, to control the combustor temperature, the flue gas has to be recycled back to the combustor and the rate has to be nearly the same as the amount of nitrogen that will be present in air in an air-fed combustion process.
Although the Anderson et al. type of process has substantial advantages compared to air-blown combustion when CO2 capture from the flue gas is required, the large amount of flue gas recycled results in high energy consumption and reduces operating reliability. As a result, this conventional method of operation needs improvement.
Furthermore, like any air combustion process, to achieve complete combustion, excess oxygen is required and, as a result, oxygen will be present in the flue gas. Yet, the presence of oxygen in the CO2 stream flue gas is undesirable for CO2 sequestration or for other utilizations. The mixture of CO2 and oxygen is also more corrosive in the presence of even small amounts of moisture. In addition, oxygen production is one of the most expensive steps in the combustion process, and discharging the flue gas with such precious oxygen is thus highly undesirable.
The circulation loop arrangement illustrated in Anderson et al. is similar to the most widely used commercial circulating fluidized beds. The aeration in the fluidized bed heat exchanger can have a negative impact on the cyclone performance and the overall solids circulation rates.
Oxy-fuel combustion in a CFB also suffers some common disadvantages of the air-fuel combustion process. For example, it requires large calcium-to-sulfur ratios to remove 90+% sulfur-containing compounds from the flue gas. Therefore for a stringent sulfur removal or near zero emission of sulfur components from the power plant, a flue gas desulfurization (FGD) device is required. Yet, capital and operating costs increase due to the addition of the FGD into the process.
There are at least two reasons for high calcium requirements to remove sulfur compounds. One is due to the atmospheric nature of operation—the sulfur compounds from the coal will be converted mostly to SO2, which has a slower reaction rate with calcium compounds. A second reason is that the particle size used in the CFB is large, and only the surface layer of the limestone particles is utilized for sulfur capture—the core of the particles has little chance to contact with the sulfur compounds in the flue gas.
For CO2 capture purposes, to produce a flue gas stream devoid of nitrogen, substantially pure oxygen is used in place of air in conventional pulverized coal (PC) boilers as disclosed in, for example, U.S. Pat. Nos. 7,282,171 and 6,918,253, and US Published Patent Applications 2009-0255450 and 2009-0257941. The methods described in these references again recycle large amounts of CO2 or flue gas to moderate and control boiler temperatures.
As described above, with the oxy-combustion CFB process, such large recycling of CO2 or flue gas to control combustion temperature leads to lower plant efficiencies and operational reliability. Also, the flue gas from the oxy-fired PC boiler described in these references contains a significant amount of excess oxygen that is necessary for boiler operation. Thus, an additional process step is needed to reduce the oxygen concentration to a relatively low ppm level to produce an essentially substantially pure CO2 stream.
Unlike CFB combustors, in-situ sulfur removal is not feasible with PC boiler. Also, the fuel grinding costs are significantly higher with combustion in PC boilers as it requires much finer fuel for substantially complete combustion of feed coal.
What is needed are better arrangements of the circulating fluidized bed loop and better operating methods to overcome the disadvantages mentioned above. It is to such systems and methods that that present invention is primarily directed. The present invention provides new arrangements for the CFB loop and the methods of operating the loop in a pressurized oxy-combustion environment.